Why a Sunflower Is Really Hundreds of Tiny Flowers

A flower is a shoot built for reproduction, with its leaves reshaped into sepals, petals, stamens and carpels. The way these parts are arranged, joined and grouped is so consistent that botanists use it to identify plant families.

This lesson covers the structure and symmetry of a flower, aestivation, placentation, cohesion and adhesion, and racemose and cymose inflorescences.

A typical flower has four whorls on a swollen thalamus — calyx, corolla, androecium and gynoecium — and is described by its symmetry and by its aestivation, the way sepals or petals are arranged in the bud.

The four whorls:

- Calyx — sepals, usually green, protecting the bud
- Corolla — petals, often brightly coloured to attract pollinators
- Androecium — stamens, each with a filament and a pollen-producing anther
- Gynoecium — carpels, each with a stigma, style and ovary containing ovules

Symmetry:

- Actinomorphic — divisible into equal halves along any radial plane; mustard, datura, chilli
- Zygomorphic — divisible into similar halves along only one plane; pea, gulmohur, bean, Cassia
- Asymmetric — cannot be divided into similar halves; Canna

Aestivation:

- Valvate — margins just touch without overlapping; Calotropis
- Twisted — each petal overlaps the next in one direction; china rose, lady's finger, cotton
- Imbricate — overlapping without a fixed direction; Cassia, gulmohur
- Vexillary — one large standard, two wings and two fused keel petals; pea and bean

An everyday example. A pea flower in a vegetable garden shows zygomorphic symmetry and vexillary aestivation, with a big standard petal on top and a boat-shaped keel below.

The substance. The position of the ovary also classifies flowers — hypogynous with a superior ovary, as in mustard; perigynous with a half-inferior ovary, as in rose; or epigynous with an inferior ovary, as in guava and cucumber.

Placentation is the arrangement of ovules inside the ovary — marginal, axile, parietal, free central or basal — while cohesion is the fusion of like parts, such as stamens with stamens, and adhesion is the fusion of unlike parts, such as stamens with petals.

Types of placentation:

- Marginal — ovules along a ridge on one side of a one-chambered ovary; pea
- Axile — ovules on a central axis in a many-chambered ovary; china rose, tomato, lemon
- Parietal — ovules on the inner wall; the ovary becomes two-chambered by a false septum; mustard, Argemone
- Free central — ovules on a central axis without septa; Dianthus, primrose
- Basal — a single ovule at the base of the ovary; sunflower, marigold

Cohesion of stamens:

- Monoadelphous — all filaments fused into one bundle; china rose
- Diadelphous — two bundles; pea
- Polyadelphous — more than two bundles; Citrus
- Syngenesious — anthers fused but filaments free; sunflower

Adhesion. Stamens attached to petals are epipetalous, as in brinjal; stamens attached to the perianth are epiphyllous, as in lily.

Gynoecium. Carpels may be free, or apocarpous, as in lotus and rose, or fused, or syncarpous, as in mustard and tomato.

An everyday example. Cutting a lemon or a tomato across the middle shows axile placentation — seeds arranged in chambers around a central axis.

The substance. Mustard looks as if it has two chambers, but its ovary is really one-chambered — the partition is a false septum, so the placentation is parietal, not axile.

In a racemose inflorescence the main axis keeps growing and flowers are borne laterally in acropetal order, with the oldest at the base, while in a cymose inflorescence the main axis ends in a flower, stops growing, and flowers open in basipetal order.

Racemose types:

- Raceme — stalked flowers on a long axis; mustard
- Spike — stalkless flowers on a long axis; Achyranthes
- Catkin — a spike of unisexual flowers; mulberry
- Spadix — a fleshy axis enclosed by a large bract called a spathe; banana, Colocasia
- Umbel — flower stalks arising from one point; coriander, carrot
- Head or capitulum — many stalkless florets on a flattened axis; sunflower, marigold

Cymose types:

- Monochasial — each axis ends in a flower with one branch below it; Heliotropium
- Dichasial — two branches below each terminal flower; jasmine
- Polychasial — several branches below each terminal flower; Calotropis

Racemose versus cymose:

- Growth of the main axis — unlimited versus limited
- Order of opening — acropetal, base to tip, versus basipetal, tip to base
- Youngest flower — at the tip versus lower down or at the sides

An everyday example. A marigold threaded into a festival garland is a capitulum — each head is a cluster of tiny florets on a flat disc.

The substance. In a capitulum the order becomes centripetal — the oldest florets sit at the rim and the youngest at the centre, which is acropetal order flattened into a disc.